On iPhone, absolutely. On Mac, I’m not sure. I know I can use the disk manager to make a new partition, install whatever OS on it I want — though, my Macs are both ARM64, so I’m quite limited there — and boot to it. I’m not sure if it’s fair to say “the bootloader is unlocked” though. Since it’s Apple’s bootloader. I don’t know if I can change it. Like on Android, when I used to mess with custom firmware ~10 years ago, we’d replace the garbage Android bootloader with TWRP (Team Win Recovery Project) and that would give us the option to make backups and to flash custom forks of Android (e.g. CyanogenMod, AOKP, etc.). And some of those bootloaders were locked down pretty tight (like HTC’s) where some were wide open (like Samsung, before Knox was a thing).
On iPhone, absolutely. On Mac, I’m not sure. I know I can use the disk manager to make a new partition, install whatever OS on it I want — though, my Macs are both ARM64, so I’m quite limited there — and boot to it. I’m not sure if it’s fair to say “the bootloader is unlocked” though. Since it’s Apple’s bootloader. I don’t know if I can change it. Like on Android, when I used to mess with custom firmware ~10 years ago, we’d replace the garbage Android bootloader with TWRP (Team Win Recovery Project) and that would give us the option to make backups and to flash custom forks of Android (e.g. CyanogenMod, AOKP, etc.). And some of those bootloaders were locked down pretty tight (like HTC’s) where some were wide open (like Samsung, before Knox was a thing).